Async::MergePoint(3) User Contributed Perl Documentation Async::MergePoint(3)NAME
"Async::MergePoint" - resynchronise diverged control flow
SYNOPSIS
use Async::MergePoint;
my $merge = Async::MergePoint->new(
needs => [ "leaves", "water" ],
);
my $water;
Kettle->boil(
on_boiled => sub { $water = shift; $merge->done( "water" ); }
);
my $tea_leaves;
Cupboard->get_tea_leaves(
on_fetched => sub { $tea_leaves = shift; $merge->done( "leaves" ); }
);
$merge->close(
on_finished => sub {
# Make tea using $water and $tea_leaves
}
);
DESCRIPTION
Often in program logic, multiple different steps need to be taken that
are independent of each other, but their total result is needed before
the next step can be taken. In synchonous code, the usual approach is
to do them sequentially.
An asynchronous or event-based program could do this, but if each step
involves some IO idle time, better overall performance can often be
gained by running the steps in parallel. A "Async::MergePoint" object
can then be used to wait for all of the steps to complete, before
passing the combined result of each step on to the next stage.
A merge point maintains a set of outstanding operations it is waiting
on; these are arbitrary string values provided at the object's
construction. Each time the "done()" method is called, the named item
is marked as being complete. When all of the required items are so
marked, the "on_finished" continuation is invoked.
For use cases where code may be split across several different lexical
scopes, it may not be convenient or possible to share a lexical
variable, to pass on the result of some asynchronous operation. In
these cases, when an item is marked as complete a value can also be
provided which contains the results of that step. The "on_finished"
callback is passed a hash (in list form, rather than by reference) of
the collected item values.
This module was originally part of the IO::Async distribution, but was
removed under the inspiration of Pedro Melo's Async::Hooks
distribution, because it doesn't itself contain anything IO-specific.
CONSTRUCTOR
$merge = Async::MergePoint->new( %params )
This function returns a new instance of a "Async::MergePoint" object.
The %params hash takes the following keys:
needs => ARRAY
Optional. An array containing unique item names to wait on. The
order of this array is not significant.
on_finished => CODE
Optional. CODE reference to the continuation for when the merge
point becomes ready. If provided, will be passed to the "close"
method.
METHODS
$merge->close( %params )
Allows an "on_finished" continuation to be set if one was not provided
to the constructor.
on_finished => CODE
CODE reference to the continuation for when the merge point
becomes ready.
The "on_finished" continuation will be called when every key in the
"needs" list has been notified by the "done()" method. It will be
called as
$on_finished->( %items )
where the %items hash will contain the item names that were waited on,
and the values passed to the "done()" method for each one. Note that
this is passed as a list, not as a HASH reference.
While this feature can be used to pass data from the component parts
back up into the continuation, it may be more direct to use normal
lexical variables instead. This method allows the continuation to be
placed after the blocks of code that execute the component parts, so it
reads downwards, and may make it more readable.
$merge->needs( @keys )
When called on an open MergePoint (i.e. one that does not yet have an
"on_finished" continuation), this method adds extra key names to the
set of outstanding names. The order of this list is not significant.
This method throws an exception if the MergePoint is already closed.
$merge->done( $item, $value )
This method informs the merge point that the $item is now ready, and
passes it a value to store, to be passed into the "on_finished"
continuation. If this call gives the final remaining item being waited
for, the "on_finished" continuation is called within it, and the method
will not return until it has completed.
EXAMPLES
Asynchronous Plugins
Consider a program using "Module::Pluggable" to provide a plugin
architecture to respond to events, where sometimes the response to an
event may require asynchronous work. A "MergePoint" object can be used
to coordinate the responses from the plugins to this event.
my $merge = Async::MergePoint->new();
foreach my $plugin ( $self->plugins ) {
$plugin->handle_event( "event", $merge, @args );
}
$merge->close( on_finished => sub {
my %results = @_;
print "All plugins have recognised $event\n";
} );
Each plugin that wishes to handle the event can use its own package
name, for example, as its unique key name for the MergePoint. A plugin
handling the event synchonously could perform something such as:
sub handle_event
{
my ( $event, $merge, @args ) = @_;
....
$merge->needs( __PACKAGE__ );
$merge->done( __PACKAGE__ => $result );
}
Whereas, to handle the event asynchronously the plugin can instead
perform:
sub handle_event
{
my ( $event, $merge, @args ) = @_;
....
$merge->needs( __PACKAGE__ );
sometime_later( sub {
$merge->done( __PACKAGE__ => $result );
} );
}
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>
perl v5.14.2 2009-07-06 Async::MergePoint(3)